We employ nonlinear autocorrelation measurements to investigate plasmon-assisted hot carrier dynamics generated in optical gold antennas. We demonstrate that surface plasmons enable a nonlinear formation of hot carriers, providing thus a unique lever to optimize the energy distribution and generation efficiency of the photo-excited charges. The temporal response of the carriers' relaxation can be controlled within a range extending from 500 fs to 2.5 ps. By conducting a quantitative analysis of the dynamics, we determine the nonlinear absorption cross-section of individual optical antennas. As such, this work provides strong insights on the understanding of plasmon-induced hot carrier generation, especially in the view of applications where the time response plays a preponderant role.

Hot carriers are energetic photo-excited carriers driving a large range of chemico-physical mechanisms. At the nanoscale, an efficient generation of these carriers is facilitated by illuminating plasmonic antennas. However, the ultrafast relaxation rate severally impedes their deployment in future hot-carrier based devices. In this paper, we report on the picosecond relaxation dynamics of hot carriers in plasmonic monocrystalline gold nanoantennas. The temporal dynamics of the hot carriers is experimentally investigated by interrogating the nonlinear photoluminescence response of the antenna with a spectrally-resolved two-pulse correlation configuration. We measure timedependent nonlinearity orders varying from 1 to 8, which challenge the common interpretation of multi-photon gold luminescence. We demonstrate that the relaxation of the photo-excited carriers depends of their energies relative to the Fermi level. We find a 60 % variation in the relaxation rate for electron-hole pair energies ranging from c.a. 0.2 to 1.8 eV. The quantitative relationship between hot carrier energy and relaxation dynamics is an important finding for optimizing hot carriersassisted processes and shed new light in the intricacy of nonlinear photoluminescence in plasmonic structures. KEY WORDS: Autocorrelation, ultrafast dynamics, nonlinear plasmonics, monocrystalline gold, nanoantennas, hot carriers, photoluminescenceWhen a light pulse illuminates a metal, photons may excite electrons to energy levels well above the Fermi level. These out-of-equilibrium electron-hole pairs, referred to as hot carriers, carry enough energy to potentially drive chemico-physical processes with applications ranging from drug delivery to nanowire growth and chemical catalysis. 1 Photons may also couple to surface plasmons, which are collective oscillations of the electronic cloud. The excitation of localized surface plasmons in metal nanoantennas enhance by many-fold the interaction cross-sections with incoming photons. 2,3 Thus, the large electromagnetic fields localized at the nanoantenna contribute to the creation of an